Before typesetting and printing, a typesetter needs to proofread the printed matter repeatedly, especially while printing newspapers and books, the typesetter needs to update and modify their contents continually. And because at each time, the modification made to the information can be only identified artificially while typesetting, this process will consume a lot of manpower and material resources, and because of comparing the content currently modified with that of last time modified is not easy for a human being, the entire proofreading process is not efficient.
Nowadays, in order to increase proofreading efficiency and reduce human errors, such that increase production efficiency, Portable Document Format (PDF for short) comparing tools are developed to proofread PDF documents. But some of the existing PDF comparing tools do not have obvious effects to the scanned PDF documents or pictures; even fail to obtain comparing result. Besides, in generally, the existing PDF comparing tools can only compare the documents according to the order of PDF pages, therefore, it is usually difficult to compare the page 1 and the page 100 of two PDF documents respectively to each other, therefore, manual intervention is needed in comparing in such situation, which reduces the comparison efficiency. Besides, the existing document comparing tools cannot replace a document page that with mistakes with a correct document page.